(1) Field of the Invention
The present invention relates to a packaging material. More particularly, the invention relates to a plastic packaging material excellent in such properties as gas barrier property, especially oxygen permeation resistance under a high humidity condition, mechanical properties and processability, which is composed of a composition comprising a plurality of ethylene-vinyl alcohol copolymers.
(2) Description of the Prior Art
As is taught in the specification of U.S. Pat. No. 3,419,654, an ethylene-vinyl alcohol copolymer obtained by saponifying an ethylene-vinyl acetate copolymer is a thermoplastic resin which is melt-extrudable and excellent in the gas permeation resistance and is broadly used as various packaging materials. This saponified ethylene-vinyl acetate copolymer shows an excellent gas barrier property under a low humidity condition such as a relative humidity of 0%. However, this polymer is defective in that the oxygen permeation coefficient (QO.sub.2) is as about 10 times under a high humidity condition such as a relative humidity of 100% and the polymer is poor in the mechanical strength and processability. Various proposals have heretofore been made on means for overcoming these defects. However, these proposals have merits and demerits.
For example, a typical method proposed for reducing the humidity dependency of the gas barrier property comprises sandwiching both the surfaces of a saponified ethylene-vinyl acetate copolymer (hereinafter referred to as "EVOH") with inner and outer layers of a moisture-resistant thermoplastic resin such as a polyolefin to form a laminate structure. According to this method, direct contact of EVOH with water or steam can be avoided, but absorption of the moisture into the EVOH layer through the polyolefin layer is caused and the gas barrier property of the EVOH layer is gradually reduced. As means for improving the steam permeation resistance of EVOH per se, there have been proposed a method in which the distribution of ethylene in EVOH is narrowed or the amount present of an ethylene homopolymer or vinyl alcohol homopolymer is reduced so that only a single endothermic peak appears in the fusion curve by a differential scanning calorimeter (see the specification of British Pat. No. 1,190,018) and a method in which EVOH is heat-treated under specific conditions so that a specific subsidiary endothermic peak based on the heat treatment appears in addition to the main endothermic peak in the fusion curve by a differential scanning calorimeter (see the specification of U.S. Pat. No. 4,082,854). However, even according to these proposals, the object of reducing the humidity dependency of the gas barrier property is not satisfactorily attained.
EVOH heretofore used for packaging materials has a molecular weight distribution (Mw/Mn) much smaller than 2.5, and it is still insufficient in mechanical properties such as impact resistance and elongation. Moreover, this EVOH is inferior to other generalpurpose resins in the processability in various molding operations such as extrusion molding and solid phase compression molding. As shown in the specification of the above-mentioned British patent, in an EVOH resin having a narrowed molecular weight distribution, the mechanical properties and processability are drastically reduced, and if this EVOH resin is subjected to draw forming or draw-blow forming, pinholes or cracks are readily formed in the resin layer.
Addition of a plasticizing substance to EVOH has been proposed to improve the mechanical properties and processability of EVOH. Indeed, these properties are improved by addition of a plasticizing substance, but this proposal is not practical because addition of a plasticizing substance results in drastic reduction of the gas barrier property of the resin.